In human anatomy, the spine is a generally flexible column that can take tensile and compressive loads. The spine also allows bending motion and provides a place of attachment for muscles and ligaments. Generally, the spine is divided into three sections: the cervical spine, the thoracic spine and the lumbar spine. The sections of the spine are made up of individual bones called vertebrae. Also, the vertebrae are separated by intervertebral discs, which are situated between adjacent vertebrae.
The intervertebral discs function as shock absorbers and as joints. Further, the intervertebral discs can absorb the compressive and tensile loads to which the spinal column may be subjected. At the same time, the intervertebral discs can allow adjacent vertebral bodies to move relative to each other a limited amount, particularly during bending, or flexure, of the spine. Thus, the intervertebral discs are under constant muscular and/or gravitational pressure and generally, the intervertebral discs are the first parts of the lumbar spine to show signs of deterioration.
The intervertebral disc functions to stabilize the spine and to distribute forces between vertebral bodies. The intervertebral disc is composed of three structures: the nucleus pulposus, the annulus fibrosis, and two vertebral end plates. These components work to absorb the shock, stress, and motion imparted to the human vertebrae. The nucleus pulposus is an amorphous hydrogel with the capacity to bind water. The nucleus pulposus is maintained within the center of an intervertebral disc by the annulus fibrosis, which is composed of highly structured collagen fibers. The vertebral end plates, composed of hyaline cartilage, separate the disc from adjacent vertebral bodies and act as a transition zone between the hard vertebral bodies and the soft disc.
Intervertebral discs may be displaced or damaged due to trauma or disease. Disruption of the annulus fibrosis may allow the nucleus pulposus to protrude into the vertebral canal, a condition commonly referred to as a herniated or ruptured disc. The extruded nucleus pulposus may press on a spinal nerve, which may result in nerve damage, back pain, numbness, muscle weakness, and, in severe cases, paralysis. Intervertebral discs may also deteriorate due to the normal aging process. As a disc dehydrates and hardens, the disc space height will be reduced, leading to instability of the spine, decreased mobility and back pain.
One way to relieve the symptoms of these conditions is by surgical removal of a portion or the entire intervertebral disc. The removal of the damaged or unhealthy disc may allow the disc space to collapse, which would lead to instability of the spine, abnormal joint mechanics, nerve damage, as well as severe back pain. Therefore, after removal of the disc, adjacent vertebrae are typically fused to preserve the disc space. Spinal fusion involves inflexibly connecting adjacent vertebrae through the use of bone grafts or metals rods. Because the fused adjacent vertebrae are prevented from moving relative to one another, the vertebrae no longer rub against each other in the area of the damaged intervertebral disc and the likelihood of continued pain and inflammation is reduced. Spinal fusion, however, is disadvantageous because it restricts the patient's mobility by reducing the spine's flexibility, and it is a relatively invasive procedure.
Attempts to overcome these problems have led researchers to investigate the efficacy of implanting an artificial intervertebral disc to replace, completely or partially, the patient's damaged intervertebral disc. Disc replacement surgery generally involves removing the disc or damaged portion thereof and placement of an artificial disc in the evacuated disc space. Some desirable attributes of a hypothetical implantable disc include axial compressibility for shock absorbance, excellent durability to avoid future replacement, minimally invasive placement of the artificial disc to reduce post-operative discomfort, and biocompatibility. Existing artificial intervertebral discs include, for example, mechanically based (e.g. comprising rotational surfaces or springs), polymer based, and biopolymer based artificial discs.
Other attempts have focused on restoring disc height in, for example, a dehydrated intervertebral disc, where a portion or all of the nucleus pulposus and a prosthetic nucleus device is implanted in the intervertebral disc space to augment or completely replace the dehydrated nucleus. Alternatively, a total disc replacement operation may be performed wherein not just the dehydrated nucleus but the entire intervertebral disc is removed and replaced with a prosthesis. However, these types of treatment often involve complex surgery and inflict a good deal of trauma on the patient, resulting in increased post-surgical recovery times and disability.
Thus, there is a need to develop new methods of intervertebral disc treatments that reduce or prevent pain and/or inflammation associated with a damaged intervertebral disc.